Table of contents

Discusses the properties of magnetic field sensors based on semiconductors such as Hall generators and magnetoresistors, and on magnetic metals, such as permalloy and the recently discovered 'giant magnetoresistance' metallic multilayers. Some emphasis is placed on the comparison between sensors made using these different technologies. Applications of magnetic field sensors in magnetic recording technology and in position sensing are discussed briefly. Typically a sensor has to detect the difference between a high and a low value of field, around an average, which is of the order of 10^-3 T in recording applications, but can exceed 0.1 T in position sensors.

The theory of laser cavity birefringence induced by insertion of two quarter-wave plates into the cavity is developed to show how the intermode frequency of a two-mode laser can be tuned through a free spectral range by relative rotation of the plates. Birefringence is demonstrated in a helium-neon laser at 633 nm using an open cavity having a tube with anti-reflection coated windows. The lock-in between modes when the intermodal frequency approaches zero was investigated and found to be smallest when the tube was placed between the plates which were aligned with axes parallel. Active frequency stabilization at a frequency separation of 50 MHz was demonstrated using the mode-balancing technique.

From a temperature correlation derived from a hyperbolic heat conduction equation due to Gaussian random process of an internal heat source, a spectral density is expressed. As a result, an effective range of a relaxation time related to finite high-speed propagation of a thermal wave in a solid is analytically estimated for real values of the spectral density and is compared with an expression derived from the conventional parabolic heat conduction equation.

A relaxation model for heat conduction and generation is presented which takes into account the finite velocity of heat propagation and the inertia of the internal heat source. Special attention is given to the effect of the source terms in the newly formulated heat conduction equation on the temperature field. The novel heat conduction equation is transformed into a dimensionless form and its special cases are considered. Several initial value problems are solved analytically for a zero-dimensional case of heat conduction and generation. It is observed that, unlike the classical hyperbolic model, the relaxation solutions generally do not tend to approach the corresponding parabolic solutions.

The authors propose a modification of their previous model which explains the effect of water on hydrous electro-rheological fluids, which are suspensions of porous particles containing water. Upon application of an electric field, water enters the gap between touching particles and adheres the particles to each other. The authors propose a model for estimating this effect and investigate by computer simulation how the aggregate structure and the viscosity depend on the water capillary force. They show that when the capillary force increases, the viscosity increases, but that the angular dependence of the adhesive force is important in the viscosity increment.

Trends in negative pressure achieved over a few thousand cavitation events were observed for ethanol, benzene and xylene in a stainless steel Berthelot tube sealed with a pre-de-gassed Ni plug. When the system was repeatedly heated and then cooled alternately over a temperature range between 60 degrees C and 10 degrees C (temperature cycle), negative pressure increased steeply for earlier cycles and levelled off eventually as in a water-metal tube system. Owing to the cavitation history effect, negative pressures of around -200 bar, the highest ever attained for organic liquids in the Berthelot method, were generated at around 15 degrees C in a useful volume ( approximately 1 cm³). It has become feasible to measure thermodynamic properties of organic liquids under negative pressure, since the pressures were up to half of the homogeneous nucleation limits of cavitation.

Experiments on the radiation from strong electrostatic double layers are performed in a double plasma machine. The spectra show that double-layer related emission exists next to global plasma phenomena. The author finds evidence for a scattering process at low frequencies, a linear acceleration emission mechanism at frequencies below the ambient plasma frequency and beam-plasma interaction at the anode side of the double layer at frequencies around 300 MHz. The brightness temperature of the emitted radiation is approximately two to three orders of magnitude higher than the temperature of the plasma.

A model has been developed of the negative glow and Faraday dark space in a Hg-Ar discharge (fluorescent lamp), which includes a self-consistently calculated axially varying electron temperature. This represents a significant advance over previously reported models where the electron temperature had to be supplied as an estimated input parameter to the model. This new model also has the advantage that the negative glow, Faraday dark space and positive column are now treated as a single continuous region with no unphysical discontinuities between them. To verify the model predictions, experimental measurements have been made using a movable Langmuir probe system. This has enabled axial plots of the electron density, electron temperature, plasma potential and electron energy distribution function to be measured. Good agreement has been obtained between the code predictions and the experimental measurements. Details of the numerical model and the experimental measurements are presented together with comparisons of the results from each.

A method for calculating the electron swarm parameters in the transverse direction to the electric field by employing a Fourier-transformed Boltzmann equation is presented. In the method, the typical transverse swarm parameter, the transverse diffusion coefficient D_T, is derived from the dispersion relation of the eigenvalue of the equation. The method is applied to methane (CH₄) and sulphur hexafluoride (SF₆). The results are compared with those from the conventional methods and it is shown that the present method is a reliable means to deduce the transverse coefficient D_T. The lateral development of the electron swarms for the model of the steady-state Townsend experiment is also discussed.

In transferred arc plasma arc welding, the plasma is not electrically neutral. Some simple mathematical models are constructed to demonstrate this fact and orders of magnitude are obtained. The relevance of these models to plasma arc welding and cutting is considered; quite long arcs can be achieved if the flow rate is sufficiently high, and an arc from a consumed tubular field emitting cathode of 75 mm length is described. In keyhole welding it is found that the current earths at the surface of the workpiece, or close to the top of the keyhole.

A Fowler-Milne technique using the relative intensity of the 746.8 nm N I line has been used to measure the plasma temperature of free-burning arcs in nitrogen at 1 atm pressure. Temperature profiles are presented for different arc currents. The maximum plasma temperature depends on the arc current and is 27000 K near the tip of a 3.2 mm diameter thoriated tungsten cathode with a 60 degrees cone angle for a 200 A, 5 mm long arc. Comparisons are made between high-current arcs burning in argon and in nitrogen.

The influence of the energy transfer efficiency on the elastic part of the electron energy distribution functions in HF sustained plasmas is investigated theoretically as well as experimentally. The gases considered are the rare gases argon, neon and helium. The different shapes of the observed electron distribution functions in their low-energy part are related to the energy transfer rate, which may be strongly energy-dependent via the momentum transfer collision frequency. The important of the Ramsauer minimum of argon for the formation of the shape of its electron distribution is pointed out. The different shapes observed for the distribution functions in helium and neon are interpreted in terms of the less energy-dependent energy transfer rates, due to the less pronounced variation of their momentum transfer cross sections.

A macroscopic model is established to describe the erosion of anode and cathode graphite electrodes. Results concerning electrode temperatures, spot radii and mass losses are presented for a current range from 0 to 10000 A. A comparison with several experimental measurements of mass losses is given.

The mechanism of the guidance effect of laser plasmas on electrical discharges has been studied. A 45 J CO₂ laser pulse produces many spherical plasmas over a gap of up to 2 m, and the laser-guided discharge is formed along the plasma by the application of a lightning impulse voltage. Unusual properties of the laser guided discharge are obtained from streak photographs of the discharge development. The laser produced plasmas guide streamers and leaders from a negative electrode more effectively than from a positive electrode. The discharge development from the negative electrode plays an important role in the guidance effect.

A combination and further development of well known isothermal two-zone models for ablation-controlled arcs is presented. With this new model it is possible to simulate the pressure in the stagnation point and the voltage inside cylindrical tubes in the high-current phase of a 50 Hz half-cycle. All necessary parameters can be determined from the measurements. The time-resolved ratio of arc cross section and nozzle cross section, the transparently radiated fraction of the arc power as well as the arc and the vapour temperature are recalculated from the measurements of arc current, pressure, voltage and mass loss. The distinction of the ablated nozzle material in a surface part and in the depth of the wall is qualitatively explainable by integrating the Kirchhoff-Planck function in two sections. The results of computations for measurements with polyethylene (PE), white, black and optimized polytetrafluorethylene (PTFE) are shown.

Measurements of the mass of gas enclosed in a quartz tube confining a burning hydrogen microwave plasma and the emitted intensities of atomic lines with axial or radial dependence are described. The microwave power was varied between about 450 and 1300 W, the gas pressure between 10 and 150 mbar. The experimental results are compared with numerical calculations. The simulations lead to three-dimensional distributions of electron densities, electron engines, field strengths, temperatures and gas compositions, and the expected mass of gas enclosed in the plasma tube. Besides some input parameters taken from the literature there remains one important fittable constant, characteristic for hydrogen, in the calculations, namely the microwave power Theta needed to sustain one electron-ion pair in the discharge. Agreement between experiments and calculations is good. The radial dependence of the light emission is used to discuss the physics behind the emission process. It is concluded that the main process is recombination of ions and electrons forming excited atoms and not excitation due to collisions with fast electrons (under the conditions investigated). Similar results were achieved in O₂, H₂/Ar and H₂/CO.

The four-prone method was used to measure the magnetoresistance of stages 2 and 5 K-FeCl₃ GICS as well as HOPG in a low magnetic field up to 5 kG at temperatures of 300, 77 and 12 K. The in-plane transverse magnetoresistance was measured. Both the transverse and longitudinal magnetoresistance were measured along the c axis. The mobility was calculated assuming a two-carrier model. The carrier concentration was calculated at 300, 77 and 12 K.

Metal-poly (p-phenylene) Schottky barriers, in which the metal and semiconductor are separated by a thin interfacial film, exhibit nearly ideal diode behaviour in the dark. The current-voltage characteristics have been studied in the range 20-300 degrees C and the current transport properties in the barriers are found to be in agreement with the diffusion theory of metal-semiconductor rectification.

The potential energy surfaces generated by microelectrodes of polynomial and interdigitated castellated geometry have been calculated for particles experiencing both positive and negative dielectrophoretic forces. The resulting forms of particle collection at these electrodes are governed by the locations of the potential energy wells, and the theoretical predictions of the modes of collection for particles experiencing positive and negative dielectrophoretic forces are verified using mixtures of viable and non-viable yeast cells, as well as of bacteria and blood cells. An important result for the interdigitated electrodes is the finding that particles trapped in potential energy wells under the action of negative dielectrophoresis can be more easily removed from the electrode structure (e.g. by fluid flow or gravitational forces) than those trapped under positive dielectrophoresis. This was verified for mixtures of bacteria and blood cells, viable and non-viable yeast cells.

Low-temperature investigation of thermal effects generally entails difficult measurements of DC temperature gradients along the sample, and require that it be mounted in a vacuum chamber. Second-sound detection of heat fluxes is an alternative and easily handled method, because the applied (heat or electric) currents are modulated (10²-10⁴ Hz) giving rise to a distribution of heat sources throughout the sample. When immersed in superfluid helium such a sample acts as a second-sound transmitter. By analysing both amplitudes and phases of the emitted waves, selectively amplified by a coupled resonator, one can obtain a lot of information about the originating phenomena. For this purpose, a suitable matrix inversion numerical scheme is presented, which should allow most practical two-dimensional problems of heat transfer involving Neumann boundary conditions to be tackled. As an illustrative example the authors describe here an experiment recently designed to localize Joule sources in type II superconductors; second-sound reveals the predicted existence of a surface Joule effect. In connection with this experiment the possibility of measuring thermomagnetic effects is discussed; the authors argue that second-sound detection is likely to be the correct way of measuring the entropy transported by moving vortices, S_d, which is likely to be responsible for a strong thermomagnetic effect in type II superconductors. It is also worth mentioning that repeated phase measurements of the thermomagnetic signal suggest a notably higher value of the Kapitza conductance at the metal-He II interface when surface heat sources are involved.

A theory of radiation-induced conductivity and space-charge build-up in disordered dielectrics during continuous irradiation with charged particles or gamma -rays is developed for arbitrary spatial distributions of carrier generation and charge deposition rates. Drift and recombination of both deposited and generated carriers are considered under the dispersive transport regime. Open-circuit conditions are analysed with one electrode (either rear or front) earthed.

Radiation-induced conductivity and space charge storage in thin films of disordered dielectrics during continuous irradiation by charged particles are considered. It is shown that the experimentally observed non-monotonic ('flash') kinetics of space charge density can be explained within the framework of a multiple trapping model of dispersive carrier transport and recombination with a non-Langevin recombination constant.

A mathematical model to predict the shading coefficients (SC) and solar rejection factor (SRF) of various types of chemically deposited solar control coatings is presented. The values of SC and SRF are presented for coatings with soar absorptance in the 50-75% range and solar reflectance in the 10-25% range for typical exterior temperature of 10-50 degrees C. Chemically deposited solar control coatings with a protective polymer coating applied on 6 mm thick sheet glass in architectural windows are seen to provide SC in the 0.3 to 0.75 range depending on the optical characteristics and exterior temperatures. A solar rejection factor of 33-56% is available from the coatings already reported.

The thermo-field emission current and Nottingham effect are calculated for a wide range of temperatures (<or=6000 K), field strengths (<or=10¹⁰ V m^-1) and work function (2-5 eV) of the emitter. The ranges of validity of known approximations are given and it is shown that the simple approximation formulae commonly used may give incorrect results to several orders of magnitude. New easy-to-use fit formulae for the emission current, the Nottingham heat and some important functions in the theory of electron emission are proposed. The main application of these approximations lies in the field of numerical modelling of the development of arc cathode spots.

The work function of an Ir(001) surface coated with a Cs overlayer is investigated by using the jellium sphere/slab model containing three layers of Ir slab, and a work function versus coverage curve much closer to the experimental one is obtained. The theoretical value of the work function at theta =0.0 (clean surface) is reduced to 5.84 eV from the former result 6.6 eV, which is now in good agreement with the experiment (5.5 eV). The minimum of the work function is found to be 1.02 eV (former value about 0.2 eV), which is also much closer to the experimental result (about 1.3 eV). The relationship between the work function and the transition of the valence electrons in the jellium sphere-vacuum region is analysed.

Atomic-scale and micro-scale images of C₆₀ (fullerene) films using scanning tunnelling microscopy (STM) are presented. The atomic configuration of a C₆₀ molecule has been clearly observed. Solvated fullerene molecules appear to pack in a hexagonal array on gold-coated freshly cleaved mica substrate, but they do not follow any long range order. STM images of ion-bombarded C₆₀ films show that the bombarded films ar smoother and possibly denser than the as-deposited films. Tribological tests on as-deposited and ion-bombarded films show that these films exhibit low friction and wear that are comparable to those of best known solid lubricants. STM images of a worn film (which has been rubbed against a steel ball) show that nearly spherical wear particles are formed which may roll in a sliding contact resulting in low friction and wear.

The problem of viscoelastic boundary layer flow over a stretching sheet and heat transfer is examined. The non-similar boundary layer equations are solved to obtain the important physical quantities such as the skin friction coefficient and heat transfer coefficient. It is found that the heat transfer coefficient decreases with the non-Newtonian parameter as well as with the frictional heating parameter.

Vogel-Fulcher-Tammann temperature dependence of sigma ( sigma varies as exp(-E_a/k(T-T₀))) was found in a KTiOPO₄ single crystal for the (001) direction. The values of T₀ and E_a were 150 K and 0.16 eV respectively. Below 150 K fast ion transport can therefore be expected to be frozen.

The electrical breakdown phenomenon in polypropylene for point-to-plane gap configuration has been investigated using a photo-optical current-measuring technique and a 760 ns rectangular high-voltage pulse generator. The waveform of prebreakdown current in polyethylene is essentially the same as that in liquid nitrogen. A linear relation to formative time delay and gap spacing is obtained for gap lengths greater than some critical value, indicating a constant propagation velocity in this region. The velocity is deduced to be 1.7 km s^-1 for a positive point at 60 kV. This value is coincident with that of polyethylene, and close to that of a longitudinal wave in polypropylene at 193 K.

The significance of the use of different analytical glow peak shapes in the application of computerized glow curve deconvolution to LiF-TLD is discussed. The contention by Gartia et al. that the use of the Podgorsak-Moran-Cameron approximation to the Randall-Wilkins model can lead to significant errors in dosimetric applications is rebutted.